The long term goal is to define the structural features of Secretory Component (SC) that are important for its participation in the secretory immune system. SC, a glycoprotein synthesized by exocrine gland epithelial cells as a transmembrane protein, serves as a receptor on the basolateral membrane of epithelial cells; here it combines with polymeric Ig. The Ig-SC complexes are endocytosed, transported across the epithelial cell in secretory vesicles, and released into secretions. At some time after interaction of the receptor form of SC and polymeric Ig, the receptor SC is apparently cleaved so that the SC released into the secretions (either as free SC or as SC bound to polymeric Ig) is significantly smaller than the transmembrane form. Rabbits apparently have 2 sizes of membrane form SC (120 kd and 95 kd) and 2 sizes of secreted form SC (80 kd and 55 kd). In addition rabbits have allotypic variants of SC (t61 and t62) which exhibit a minor size difference of 1-2 kd. The minor size differences and the antigenic differences of t61 and t62 allotypes are present in both the 80 kd and 55 kd forms of secreted SC. The studies described in this proposal will take advantage of the multiple forms of rabbit SC in the secretions to compare structural differences and functional differences and to identify structure-function correlates. Comparative structural studies will be done by peptide mapping; both High Performance Liquid Chromatography (HPLC) and 2-D thin layer techniques will be used. In addition, large quantities of the 80 kd form and the 55 kd form of t61 SC will be prepared for amino acid sequence studies. Comparative functional studies will be done by two means: 1) relative binding abilities of the various forms of SC for dimeric IgA will be determined and 2) relative capacities of the various forms of SC to protect IgA from proteolytic degradation will be assessed. Both non-covalent and covalent interactions of SC and dimeric IgA will be studied, reactants and products for binding studies and for proteolytic degradation studies will be analyzed by HPLC and by immunochemical techniques. Genetic studies will be extended to further characterize an additional genetic variant (t63) of rabbit SC which appears to be controlled by a 3rd allele at the t-locus. Several rabbit colonies will be surveyed to determine the frequencies of t61, t62, and t63 and the frequencies of additional variants. Identification of variants will be done by immunochemical (radioimmunoassay) and physicochemical (SDS-PAGE) techniques.